Semiconductor and noble metal catalysts are often employed to promote high efficiency chemical processes including both catalytic and photocatalytic reactions. Before such chemical processes can be efficiently implemented, it is often necessary to immobilize or fixate a selected catalyst on a particular support structure.
Immobilized semiconductors and noble metals have been shown to be effective catalysts for a variety of heterogeneous and photolytic reactions such as photogeneration of hydrogen, photocatalytic oxidation, and detoxification of hazardous wastes. Moreover, immobilization of semiconductors and noble metals is especially useful from the process engineering point of view because it simplifies and facilitates the separation of the catalyst from the reaction medium and reaction products.
Several approaches have been used in the past to immobilize a selected catalyst on various support structures. For example, the most widely used method of immobilizing semiconductors involves soaking or impregnating a selected support surface with an aqueous solution containing a semiconductor suspension. The surface is then dried to achieve immobilization of the semiconductor on the surface. More particularly, Al-Ekabi and Serpone teach the introduction of a suspension of TiO.sub.2 particles into a coiled glass support so as to cover the entire internal surface of the glass support. The suspension was then evaporated to dryness under vacuum to immobilize the TiO.sub.2 particles on the glass support. (See Al-Ekabi et al., "Kinetic Studies In Heterogeneous Photocatalysis", J. Phys. Chem, vol. 92, pp. 5726-5731 (1988).)
In U.S. Pat. No. 4,861,484, Lichtin et al. teach a similar technique which involves mixing approximately 1 g of TiO.sub.2 powder with about 20 ml of water and applying the resulting mixture onto the internal surface of a glass cylinder which served as a reaction cell. The glass cylinder was rotated by hand to evenly coat the inner surface of the cylinder. Air evaporation of water was then conducted to dry the cylinder. Unfortunately, such soaking-drying methods are unlikely to result in sufficient binding between the catalyst and the surface. This is mainly due to weak Van der Waals type interactions between the semiconductor catalyst and the glass surface. This weak interaction eventually leads to loss of the active phase especially when the immobilized structure is subjected to very turbulent flow regimes.
Another immobilization technique is disclosed by Kuczynski and Thomas in J. Phys. Chem, vol. 89, pp. 2720-2722 (1985). That technique involves multiple preparation stages including soaking the support surface in an aqueous solution of an appropriate metal salt and subsequent formation of the semiconductor directly on the support surface by an ion exchange reaction. More specifically, the technique involves the preparation of CdS on porous glass by soaking the clean porous glass surface in CdCl.sub.2 solution before drying it under vacuum to remove water, thereby leaving the cadmium salt on the glass. The impregnated glass was then immersed in Na.sub.2 S solution to complete the immobilization process.
Still another multiple preparation stage immobilization technique is disclosed by Ueno et al., J. Phys. Chem, vol. 89, pp. 3828-3833 (1985). Ueno et al. teach immobilizing CdS and ZnS on supports such as silica powder, thin films of perfluorocarbon sulfonate ionomers, frosted glass, weighing paper, and a polyamide mesh. In each of these instances, the support was soaked in Cd(NO.sub.3).sub.2 or Zn(NO.sub.3).sub.2 solution and then placed in an H.sub.2 S saturated aqueous solution. Unfortunately, this particular approach exhibited some disadvantages. For example, this approach immobilizes some types of semiconductors better than others. This method works well with sulfide type semiconductors but not as well with oxide type semiconductors. The authors also reported that sonication removed ZnS-CdS particles from glass and paper supports. This demonstrates a weakness in the binding action between the semiconductor and these support surfaces. However, CdS immobilized perfluorocarbon sulfonate ionomer films (Nafion films) exhibit excellent binding between the semiconductor and the polymeric support surface due to the ionic bonding between Cd.sup.2+ and the cation exchange resin. Nafion is a trademark of E. I. du Pont de Nemours & Co., Inc.
Several high temperature methods of immobilization including a pyrolysis stage are also known. For example, Serpone et al. describe impregnation of TiO.sub.2 on glass beads by the high temperature thermal decomposition of titanium (IV) alkoxides. (See Solar Energy Materials, Vol. 14, pp. 121-127 (1986).) Augugliaro et al. have also reported a method of preparing alumina supported iron doped TiO.sub.2. (See Int. J. Hydrogen Energy, vol. 7, pp. 845-849 (1982).) In that method, alumina was pore volume impregnated by an iron sulfate containing TiCl.sub.3 solution. After mixing and stirring, the mixture was slowly heated up to 700.degree. C. and then maintained at this temperature for twenty-four hours. Moreover, Smestad et al. disclose a chemical spray pyrolysis (CSP) method for producing thin film semiconducting iron oxides and iron pyrite. (See Solar Energy Materials, vol. 20, pp 149-165 (1990). In that method, Fe.sub.3 O.sub.4 and Fe.sub.2 O.sub.3 films were prepared by CSP of FeCl.sub.2 and FeCl.sub.3, respectively. The sprayed oxide layers were then deposited on Schott AF45 glass. Pyrite layers were obtained by the reaction of Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4 films with gaseous sulfur heated to 350.degree. C. for up to two hours. Unfortunately, this method is not generally applicable to all types of semiconductors. For example, this method does not permit immobilization of complex oxide type semiconductors such as SrTiO.sub.3 on temperature resistant supports. Moreover, With this CSP technique, it is difficult to control the formation of the specific form of the semiconductor structure on the support surface.
In addition, many vapor deposition (VD) methods are known for immobilizing metals on selected support structures. For example, in Solar Energy Materials, vol. 20, pp 149-165 (1990), Smestad et al. describe the preparation of iron oxide layers by evaporating iron on the support material with subsequent oxidation in an N.sub.2 /O.sub.2 gas flow at 350.degree. C. The evaporated layers were deposited on a 0.025 mm Kapton (Kapton is a trademark of DuPont) polyimide support structure and AF45 glass. This VD method is limited to semiconductors as the deposited material and is further limited to supports which are stable at high temperatures in a hard vacuum.
None of the immobilization methods described above are universal in that none of these methods provide a reliable way of fixing a wide range of different types of semiconductors (and modifications thereof) and noble metals onto a wide range of organic and inorganic solid support surfaces.